The invention is based on a priority application EP 03 290 921.0 which is hereby incorporated by reference.
The invention is related to a Raman amplifying device and in more detail to a Raman amplifying device for amplifying signals (S1, S2 . . . Sn) with wavelengths λS1, λS2 . . . λSn comprising an optical path, pump sources (P1,P2 . . . PN) for generating a plurality of Raman pump signals (λ1,λ2, . . . λN) for backward pumping and means for coupling (21, 22, . . . 2N)) the plurality of Raman pump signals into the optical path, wherein the plurality of optical Raman pump signals are time-division multiplexed by multiplexing controlling means (41,42, . . . 4N). The invention is also related to a method for time multiplexing a plurality of Raman pump signals in an amplifying device.
Raman optical amplifiers are important components in optical communication systems. Optical fiber communication systems are beginning to achieve their great potential for the rapid transmission of vast amounts of information. In essence, an optical fiber system comprises a source of information-carrying optical signals, an optical fiber transmission line for carrying the optical signals and a receiver for detecting the optical signals and demodulating the information they carry. The signals are typically within a wavelength range favorable for propagation within silica fibers, and preferably comprise a plurality of wavelength distinct channels within that range.
Despite significant progress in reducing the attenuation characteristics of optical fibers, signals transmitted through them are attenuated by the cumulative and combined effect of absorption and scattering. Consequently long distance transmission requires periodic amplification.
One approach to optical amplification utilizes Raman effect amplification. In the Raman effect amplification, light traveling within a medium is amplified by the presence of lower wavelength pump light traveling within the same medium. The gain spectrum of a silica fiber pumped by a monochromatic Raman pump exhibits maximum gain when the signal to be amplified is at a frequency approximately 13 THz lower than the frequency of the Raman pump. The frequency (or wavelength) difference between the pump and the frequency (or wavelength) of maximum gain is often referred to as the Stokes shift, and the amplified signal is referred to as the Stokes wave. Use of a pump that is detuned from the signals by about one Stokes shift (½ the Stokes shift to 3/2 the shift) is referred to as first-order Stokes pumping.
It has also been observed that the gain is significantly larger for a co-polarized signal and pump. This polarization sensitivity can be eliminated if the pump is depolarized, polarization-scrambled or composed of two equally powerful polarized pumps that are polarization multiplexed.
Raman amplifiers can be categorized as either distributed or discrete. In distributed amplifiers, the transmission fiber itself is used as the gain medium. In discrete amplifiers, a separate fiber, typically optimized for Raman amplification, is used as the gain fiber. While the discrete amplifier gain fiber may be kilometers in length it is typically spooled at one location and not used to transfer information from one location to another. The term “Raman amplifier”, as used herein, refers to both the pump and the gain medium.
A difficulty with conventional Raman amplifiers is that they are typically critically dependent on power sensitive components subsequent (downstream) to the pump. Often Raman pump sources utilize a plurality of sources to establish a high power first order pump (>100 mW) and an immediate downstream multiplexing component to combine the outputs into a pump with a wide, flat bandwidth. In typical amplifiers, the pump power is generated by an array of high power semiconductor pump lasers that are followed by wavelength-division multiplexers.
The problem with this amplifiers is the harmful interaction of the different pump source wavelengths traveling along the fiber.
With the recent ascendance of dense WDM and the commercial availability of several hundred mW output semiconductor pump laser the use of Raman amplifiers comes a good opportunity. Raman gain is now well used to overcome noise and non-linear penalties and to adapt the gain band according the available pump bands.
The EP 1 148 666 discloses a method to pump a Raman amplifier. The method involves time division multiplexing of combined pump wavelengths to attain broad Raman gain and to avoid the interaction of the different pump sources. This multiplexing scheme produces a flat gain spectrum. In the EP 1148 666 the modulation frequency that is required to keep the signal unaffected is specified with a few MHz.
However, Double Rayleigh Scattering noise (DRS) increases dramatically if no more precaution is taken. Indeed, as the Rayleigh noise co-propagates with the pump between the first and second backscattering, it experiences high variations of gain in a dB scale. As a result the average amount of DRS noise is much higher with a modulated pump than with a continuous pump that would give the same on-off gain.
The problem to solve is to realize pump modulation without rising high DRS penalty.